/*
 * Copyright (C) 2008 The Android Open Source Project
 * Copyright (C) 2011 Daniel Himmelein
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef MINDROID_OS_ASYNCTASK_H_
#define MINDROID_OS_ASYNCTASK_H_

#include <mindroid/os/Handler.h>
#include <mindroid/util/concurrent/SerialExecutor.h>
#include <mindroid/util/concurrent/ThreadPoolExecutor.h>
#include <mindroid/util/concurrent/atomic/AtomicBoolean.h>
#include <mindroid/lang/IllegalStateException.h>

namespace mindroid {

class AsyncTaskBase :
        public Object {
public:
    static sp<Executor> SERIAL_EXECUTOR;
    static sp<Executor> THREAD_POOL_EXECUTOR;
};

/**
 * <p>AsyncTask enables proper and easy use of the UI thread. This class allows to
 * perform background operations and publish results on the UI thread without
 * having to manipulate threads and/or handlers.</p>
 *
 * <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
 * and does not constitute a generic threading framework. AsyncTasks should ideally be
 * used for short operations (a few seconds at the most.) If you need to keep threads
 * running for long periods of time, it is highly recommended you use the various APIs
 * provided by the <code>java.util.concurrent</code> package such as {@link Executor},
 * {@link ThreadPoolExecutor} and {@link FutureTask}.</p>
 *
 * <p>An asynchronous task is defined by a computation that runs on a background thread and
 * whose result is published on the UI thread. An asynchronous task is defined by 3 generic
 * types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>,
 * and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
 * <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p>
 *
 * <div class="special reference">
 * <h3>Developer Guides</h3>
 * <p>For more information about using tasks and threads, read the
 * <a href="{@docRoot}/guide/components/processes-and-threads.html">Processes and Threads</a>
 * developer guide.</p>
 * </div>
 *
 * <h2>Usage</h2>
 * <p>AsyncTask must be subclassed to be used. The subclass will override at least
 * one method ({@link #doInBackground}), and most often will override a
 * second one ({@link #onPostExecute}.)</p>
 *
 * <p>Here is an example of subclassing:</p>
 * <pre class="prettyprint">
 * private class DownloadFilesTask extends AsyncTask&lt;URL, Integer, Long&gt; {
 *     protected Long doInBackground(URL... urls) {
 *         int count = urls.length;
 *         long totalSize = 0;
 *         for (int i = 0; i < count; i++) {
 *             totalSize += Downloader.downloadFile(urls[i]);
 *             publishProgress((int) ((i / (float) count) * 100));
 *             // Escape early if cancel() is called
 *             if (isCancelled()) break;
 *         }
 *         return totalSize;
 *     }
 *
 *     protected void onProgressUpdate(Integer... progress) {
 *         setProgressPercent(progress[0]);
 *     }
 *
 *     protected void onPostExecute(Long result) {
 *         showDialog("Downloaded " + result + " bytes");
 *     }
 * }
 * </pre>
 *
 * <p>Once created, a task is executed very simply:</p>
 * <pre class="prettyprint">
 * new DownloadFilesTask().execute(url1, url2, url3);
 * </pre>
 *
 * <h2>AsyncTask's generic types</h2>
 * <p>The three types used by an asynchronous task are the following:</p>
 * <ol>
 *     <li><code>Params</code>, the type of the parameters sent to the task upon
 *     execution.</li>
 *     <li><code>Progress</code>, the type of the progress units published during
 *     the background computation.</li>
 *     <li><code>Result</code>, the type of the result of the background
 *     computation.</li>
 * </ol>
 * <p>Not all types are always used by an asynchronous task. To mark a type as unused,
 * simply use the type {@link Void}:</p>
 * <pre>
 * private class MyTask extends AsyncTask&lt;Void, Void, Void&gt; { ... }
 * </pre>
 *
 * <h2>The 4 steps</h2>
 * <p>When an asynchronous task is executed, the task goes through 4 steps:</p>
 * <ol>
 *     <li>{@link #onPreExecute()}, invoked on the UI thread before the task
 *     is executed. This step is normally used to setup the task, for instance by
 *     showing a progress bar in the user interface.</li>
 *     <li>{@link #doInBackground}, invoked on the background thread
 *     immediately after {@link #onPreExecute()} finishes executing. This step is used
 *     to perform background computation that can take a long time. The parameters
 *     of the asynchronous task are passed to this step. The result of the computation must
 *     be returned by this step and will be passed back to the last step. This step
 *     can also use {@link #publishProgress} to publish one or more units
 *     of progress. These values are published on the UI thread, in the
 *     {@link #onProgressUpdate} step.</li>
 *     <li>{@link #onProgressUpdate}, invoked on the UI thread after a
 *     call to {@link #publishProgress}. The timing of the execution is
 *     undefined. This method is used to display any form of progress in the user
 *     interface while the background computation is still executing. For instance,
 *     it can be used to animate a progress bar or show logs in a text field.</li>
 *     <li>{@link #onPostExecute}, invoked on the UI thread after the background
 *     computation finishes. The result of the background computation is passed to
 *     this step as a parameter.</li>
 * </ol>
 *
 * <h2>Cancelling a task</h2>
 * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking
 * this method will cause subsequent calls to {@link #isCancelled()} to return true.
 * After invoking this method, {@link #onCancelled(Object)}, instead of
 * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])}
 * returns. To ensure that a task is cancelled as quickly as possible, you should always
 * check the return value of {@link #isCancelled()} periodically from
 * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p>
 *
 * <h2>Threading rules</h2>
 * <p>There are a few threading rules that must be followed for this class to
 * work properly:</p>
 * <ul>
 *     <li>The task instance must be created on a looper thread.</li>
 *     <li>{@link #execute} must be invoked on a looper thread.</li>
 *     <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute},
 *     {@link #doInBackground}, {@link #onProgressUpdate} manually.</li>
 *     <li>The task can be executed only once (an exception will be thrown if
 *     a second execution is attempted.)</li>
 * </ul>
 *
 * <h2>Memory observability</h2>
 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following
 * operations are safe without explicit synchronizations.</p>
 * <ul>
 *     <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them
 *     in {@link #doInBackground}.
 *     <li>Set member fields in {@link #doInBackground}, and refer to them in
 *     {@link #onProgressUpdate} and {@link #onPostExecute}.
 * </ul>
 */
template<typename Params, typename Progress, typename Result>
class AsyncTask :
        public AsyncTaskBase {
public:
    AsyncTask() :
            mHandler(new InternalHandler()),
            mRunnable(new WorkerRunnable(this)) {
    }

    virtual ~AsyncTask() = default;

    AsyncTask(const AsyncTask&) = delete;
    AsyncTask& operator=(const AsyncTask&) = delete;

    sp<AsyncTask<Params, Progress, Result>> execute(Params params) {
        return executeOnExecutor(SERIAL_EXECUTOR, params);
    }

    sp<AsyncTask<Params, Progress, Result>> executeOnExecutor(const sp<Executor>& executor, Params params) {
        if (mStatus != Status::PENDING) {
            switch (mStatus) {
            case Status::PENDING:
                return nullptr;
            case Status::RUNNING:
                throw IllegalStateException("Cannot execute task: the task is already running");
            case Status::FINISHED:
                throw IllegalStateException("Cannot execute task: the task has already been executed (a task can be executed only once)");
                return nullptr;
            }
        }

        mSelf = this;
        mStatus = RUNNING;
        mExecutor = executor;

        onPreExecute();

        mRunnable->mParams = params;
        mExecutor->execute(mRunnable);

        return this;
    }

    /**
     * <p>
     * Attempts to cancel execution of this task. This attempt will fail if the task has already
     * completed, already been cancelled, or could not be cancelled for some other reason. If
     * successful, and this task has not started when <tt>cancel</tt> is called, this task should
     * never run. If the task has already started, then the <tt>mayInterruptIfRunning</tt> parameter
     * determines whether the thread executing this task should be interrupted in an attempt to stop
     * the task.
     * </p>
     *
     * <p>
     * Calling this method will result in {@link #onCancelled(Object)} being invoked on the UI
     * thread after {@link #doInBackground(Object)} returns. Calling this method guarantees that
     * {@link #onPostExecute(Object)} is never invoked. After invoking this method, you should check
     * the value returned by {@link #isCancelled()} periodically from
     * {@link #doInBackground(Object)} to finish the task as early as possible.
     * </p>
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this task should be
     * interrupted; otherwise, in-progress tasks are allowed to complete.
     *
     * @return <tt>false</tt> if the task could not be cancelled, typically because it has already
     * completed normally; <tt>true</tt> otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    bool cancel() {
        if (!isCancelled() && mStatus == RUNNING) {
            mCancelled->set(true);
            bool cancelled = mExecutor->cancel(mRunnable);
            if (cancelled) {
                sp<AsyncTaskResult> result = new AsyncTaskResult(this);
                result->setResult(nullptr);
                mHandler->obtainMessage(MESSAGE_POST_RESULT, result)->sendToTarget();
            }
            return true;
        } else {
            return false;
        }
    }

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed normally. If you are
     * calling {@link #cancel(boolean)} on the task, the value returned by this method should be
     * checked periodically from {@link #doInBackground(Object)} to end the task as soon as
     * possible.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     *
     * @see #cancel(boolean)
     */
    bool isCancelled() {
        return mCancelled->get();
    }

protected:
    /**
     * Override this method to perform a computation on a background thread. The specified
     * parameters are the parameters passed to {@link #execute} by the caller of this task.
     *
     * This method can call {@link #publishProgress} to publish updates on the Handler thread.
     *
     * @param params The parameters of the task.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPreExecute()
     * @see #onPostExecute
     * @see #publishProgress
     */
    virtual Result doInBackground(Params params) = 0;

    /**
     * Runs on the Handler thread before {@link #doInBackground}.
     *
     * @see #onPostExecute
     * @see #doInBackground
     */
    virtual void onPreExecute() {
    }

    /**
     * <p>
     * Runs on the Handler thread after {@link #doInBackground}. The specified result is the value
     * returned by {@link #doInBackground}.
     * </p>
     *
     * <p>
     * This method won't be invoked if the task was cancelled.
     * </p>
     *
     * @param result The result of the operation computed by {@link #doInBackground}.
     *
     * @see #onPreExecute
     * @see #doInBackground
     * @see #onCancelled(Object)
     */
    virtual void onPostExecute(Result result) {
    }

    /**
     * Runs on the Handler thread after {@link #publishProgress} is invoked. The specified values are the
     * values passed to {@link #publishProgress}.
     *
     * @param values The values indicating progress.
     *
     * @see #publishProgress
     * @see #doInBackground
     */
    virtual void onProgressUpdate(Progress values) {
    }

    /**
     * <p>
     * Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.
     * </p>
     *
     * <p>
     * The default implementation simply invokes {@link #onCancelled()} and ignores the result. If
     * you write your own implementation, do not call <code>super.onCancelled(result)</code>.
     * </p>
     *
     * @param result The result, if any, computed in {@link #doInBackground(Object[])}, can be null
     *
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    virtual void onCancelled(Result result) {
        onCancelled();
    }

    /**
     * <p>
     * Applications should preferably override {@link #onCancelled(Object)}. This method is invoked
     * by the default implementation of {@link #onCancelled(Object)}.
     * </p>
     *
     * <p>
     * Runs on the Handler thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object)} has finished.
     * </p>
     *
     * @see #onCancelled(Object)
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    virtual void onCancelled() {
    }

    /**
     * This method can be invoked from {@link #doInBackground} to publish updates on the UI thread
     * while the background computation is still running. Each call to this method will trigger the
     * execution of {@link #onProgressUpdate} on the UI thread.
     *
     * {@link #onProgressUpdate} will note be called if the task has been canceled.
     *
     * @param values The progress values to update the UI with.
     *
     * @see #onProgressUpdate
     * @see #doInBackground
     */
    void publishProgress(Progress values) {
        if (!isCancelled()) {
            sp<AsyncTaskResult> result = new AsyncTaskResult(this);
            result->setProgress(values);
            mHandler->obtainMessage(MESSAGE_POST_PROGRESS, result)->sendToTarget();
        }
    }

    Params params() {
        return mRunnable->mParams;
    }

    Result result() {
        return mRunnable->mResult;
    }

private:
    void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = FINISHED;
        mRunnable.clear();
        mSelf.clear();
    }

    class InternalHandler :
            public Handler {
    public:
        virtual void handleMessage(const sp<Message>& message) {
            sp<AsyncTaskResult> result = object_cast<AsyncTaskResult>(message->obj);
            switch (message->what) {
            case MESSAGE_POST_RESULT: {
                result->mTask->finish(result->mResult);
                break;
            }
            case MESSAGE_POST_PROGRESS: {
                result->mTask->onProgressUpdate(result->mProgress);
                break;
            }
            }
        }
    };

    class WorkerRunnable :
            public Runnable {
    public:
        WorkerRunnable(const sp<AsyncTask<Params, Progress, Result>>& task) :
                mTask(task) {
        }

        virtual void run() {
            sp<AsyncTaskResult> result = new AsyncTaskResult(mTask);
            result->setResult(mTask->doInBackground(mParams));
            mTask->mHandler->obtainMessage(MESSAGE_POST_RESULT, result)->sendToTarget();
        }

        Params mParams;

    private:
        sp<AsyncTask<Params, Progress, Result>> mTask;

        friend class InternalHandler;
    };

    class AsyncTaskResult :
            public Object {
    public:
        AsyncTaskResult(const sp<AsyncTask<Params, Progress, Result>>& task) :
                mTask(task) {
        }

        void setResult(const Result& result) {
            mResult = result;
        }

        void setProgress(const Progress& progress) {
            mProgress = progress;
        }

    private:
        sp<AsyncTask<Params, Progress, Result>> mTask;
        Result mResult;
        Progress mProgress;

        friend class InternalHandler;
    };

    sp<Executor> mExecutor;
    sp<InternalHandler> mHandler;
    sp<WorkerRunnable> mRunnable;
    sp<AtomicBoolean> mCancelled = new AtomicBoolean();
    sp<AsyncTask<Params, Progress, Result>> mSelf;

    /**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    enum Status {
        /**
         * Indicates that the task has not been executed yet.
         */
        PENDING,
        /**
         * Indicates that the task is running.
         */
        RUNNING,
        /**
         * Indicates that {@link AsyncTask#onPostExecute} has finished.
         */
        FINISHED,
    };

    /**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    Status mStatus = PENDING;

    static const int32_t MESSAGE_POST_RESULT = 1;
    static const int32_t MESSAGE_POST_PROGRESS = 2;
};

} /* namespace mindroid */

#endif /* MINDROID_OS_ASYNCTASK_H_ */
